Most control systems use Proportional-Integral-Derivative (PID) algorithms for controlling DO, basin air flow distribution, and blower pressure or flow. These algorithms are prone to hunting - the cyclic fluctuation of the controlled variable. Fluctuations that oscillate the DO several mg/L above and below the target DO concentration are common.
Sizing, selection, and adjusting control valves often causes confusion for process and control system designers. Improper valve application can cause operating problems for plant staff and waste blower power. Basing the airflow control system design on fundamental principles will improve valve and control system performance.
This article aims to discuss the various technologies of equipment that could be presented to a wastewater operator faced with a temporary need for blower air and to help the operator understand the impact (both monetarily through a “Total Cost of Rental” approach and environmentally) of their decisions.
Real world blower applications rarely operate at steady state design conditions. There are a variety of reasons for this. Designs usually include a margin of safety to accommodate unforeseen conditions. Typically, the process demand itself is variable, requiring a corresponding ability to modulate the blower flowrate.
A small site located within a floodplain, prone to erosion, and currently occupied by an existing in-service wastewater treatment facility is not at the top of any engineer’s list for a desirable site to expand a wastewater treatment plant or reclamation facility. However, these challenges created opportunity for specialized solutions during the design of the facility expansion; in particular, in designing the aeration and digester blower system.
Most electric utilities offer customer incentives for implementing energy conservation measures (ECMs) Incentive programs pay customers to use less energy. In some cases they are mandated by legislation and in others the incentives are driven by the utility’s desire to avoid building new generating capacity. Some incentives are based on reduced energy use (kWh) and some are based on lower peak demand (kW).
When the plant’s original aeration blowers became costly to operate and newer technology offered the promise of energy-savings, Fuqua took decisive action and replaced the older blowers with high-speed turbo blowers. As a result, the plant saves ratepayers approximately $30,000 per year in energy costs and bolsters the plant’s ability to maintain uptime and achieve extremely clean effluent.
Optimized intake filters can save thousands of dollars annually in energy savings and may not require construction crews, engineering bids, or grant applications. The benefits can also be realized for treatment plants of all sizes. Described below are examples of three wastewater treatment plants that upgraded filters and came out ahead: a small rural operation with positive displacement (PD) blowers, a suburban plant using multi-stage blowers and a large urban plant that had already upgraded to airfoil bearing high-speed turbo blowers.
One need look no further than the treatment plant’s digester project upgrade to see the value of the plant’s forward-thinking approach at work. The upgrade involved the replacement of five Positive Displacement (PD) blowers with four, high-speed Inovair integrally geared blowers for aerating the digesters. With fewer – and smaller – blowers the plant saves tens of thousands of dollars in energy costs per year. The blowers also offer flexibility to cost-effectively adapt to the need for increased water treatment in the future.
Aeration blower upgrades may be part of a total plant upgrade and minimizing energy consumption is a critical consideration. Blower replacements are also a common Energy Conservation Measure (ECM) in cost-reduction programs.
Julie Gass, P.E., is a Lead Mechanical Process Engineer at Black & Veatch and an industry veteran with extensive experience in mechanical equipment in wastewater treatment plants. She also served on the American Society of Mechanical Engineers (ASME) Committee responsible for ASME PTC 13, Wire-to-Air Performance Test Code for Blower Systems, which is the performance test code published in October 2019 for all blower technologies. Blower & Vacuum Best Practices Magazine interviewed Gass to gain her views on aeration blowers, PTC 13, and the firm’s rigorous specification process to ensure treatment plants get the blower best suited for their application.